1. Field of the Invention
This invention relates to frequency tunable magnetrons, and more particularly, to a tuning mechanism utilizing spring loading to overcome frictional forces which vary with temperature during testing.
2. Description of Related Art
Crossed-field tubes, such as magnetrons, are commonly used to generate RF or microwave electromagnetic energy for assorted applications such as radar. A magnetron commonly has a cylindrically shaped cathode coaxially disposed within an anode comprising a plurality of radially extending anode vanes. Alternating ones of the vanes may be electrically connected together by straps. The space between the cathode surface and the anode provides a cavity, and a potential is applied between the cathode and the anode forming an electric field across the cavity. A magnetic field is also provided in the cavity perpendicular to the electric field. Electrons emitted thermionically from the cathode surface are caused to orbit around the cathode in the cavity due to the crossed electric and magnetic fields, and the orbiting electrons interact with an RF electromagnetic wave moving on the anode. The electrons give off energy to the moving RF wave, thus producing a high power microwave output signal.
It is useful to provide a magnetron having a frequency which can be tuned, in order to calibrate the magnetron for a desired operational frequency. Many techniques are used for tuning magnetrons, and typically employ changes in the capacitance or the inductance of the magnetron cavity. An example of a prior art tuning device for a coaxial magnetron is found in U.S. Pat. No. 4,531,104 for TUNABLE MAGNETRON OF THE COAXIAL-VACUUM TYPE, which issued Jul. 23, 1985, by Schaeffer.
In a magnetron having alternating ones of the anode vanes electrically connected together by a first and a second strap, respectively, tuning may be achieved by altering the capacitance between the straps. In such a magnetron, the anode vanes are configured to define a space between the straps, and a nonconductive tuning element, such as a ceramic cylinder, moved into a portion of the space. The capacitance between the straps is altered by the introduction of the tuning element, and accordingly, a resonant characteristic of the cavity is changed and tuning of the resonant frequency of the magnetron can be accomplished. In practice, relatively small changes in tuning element position can yield substantial changes in resonant frequency; in one such magnetron, for every 0.001 inch that the tuning element is inserted between the straps, the resonant frequency changes 18.6 MHz. Thus, accurate control of the tuning element position is critical.
There are various techniques for positioning tuning elements within the resonant cavity. The tuning element can be disposed at an end of a tuner shaft having a threaded outside diameter. A vertical gear having a threaded inside diameter concentrically engages the threaded outside diameter of the tuner shaft, such that the vertical gear and tuner shaft share a common rotational axis. Rotation of the vertical gear causes the tuner shaft to move in the axial direction. A worm gear engages the vertical gear, and rotation of the worm gear translates into axial movement of the tuner shaft. The worm gear can then be manually rotated to achieve the desired position for the tuning element, and the desired resonant frequency. Alternatively, the movement can be controlled by an electromechanical device, such as a motor.
A drawback of this type of tuning mechanism is that it is susceptible to positional variations due to environmental considerations. During qualification and acceptance of magnetrons for governmental use, the magnetrons are exposed to wide temperature changes and mechanical vibrations, commonly known as "shake and bake" testing. Ideally, the tuning element should maintain its position throughout the test range; according to a government specification, the frequency can not deviate more that 2 MHz from the original reading, thus the tuning element position could not deviate more than 0.000107075 inch from its original setting. In actual practice, however, it is difficult to control the position with the required degree of accuracy.
The position fluctuation is due, in part, to space provided between a threaded shaft sleeve coupled to the tuner shaft, and a bellows retainer. The engagement between the shaft sleeve and the bellows retainer must be tight, otherwise any play between the two members would translate to undesired movement of the tuning element. If the coupling between the two members is too tight, however, the torque required to overcome the frictional forces between the members becomes too great to permit fine adjustment of the tuning element position.
Accordingly, it would be desirable to provide a tuning mechanism for a magnetron that provides adequate frequency stability without the need for excessive torque to operate the moveable tuning element.